Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 79

conterminous United States. Most of these aquifers are composed of sedimentary rock and
underlie a majority of the country's major urban areas.

3.5.1 Aquifer Types and Basic Structure

There are two types of aquifers, confined and unconfined, based on the presence or
absence of a water table. A water table or phreatic surface is the upper extent of the zone
of saturation in direct contact with atmospheric air pressure through void spaces in the
overlying geologic material. An unconfined aquifer , therefore, is defined as a saturated
geologic material where the surface of the saturated layer is equal to atmospheric pressure
(Heath 1983). The depth to the water table surface can range from just a few meters or less
in humid regions to more than 305 m (1000 ft) in dry desert regions. Unconfined aquifers
are usually the first saturated waters encountered beneath the surface. If this water is of
limited areal extent, it is commonly known as a perched aquifer or perched water depend-
ing on the amount and extent of saturated materials.
Perched water or perched aquifers are commonly encountered in urban environments.
Many building foundations, sewers, and roads are backfilled with materials such as sand
and gravel with higher permeability (capable of being penetrated by water) than the
indigenous soils often composed of clay. In these cases, water eventually saturates the
more permeable materials surrounding the foundation and perched water develops. In a
pure geologic sense, this underground configuration of water does not fit the definition of
an aquifer because it is not a mappable geologic unit, does not occur in a naturally occur-
ring formation, and exists because of anthropogenic activities.
Conined aquifers are permeable geologic formations or strata bounded above and
below by relatively impervious geologic material consisting of clay layers or other similar
materials and contain water at pressure greater than atmospheric (Heath 1983). These con-
fining layers or strata separate the higher permeability aquifer material from direct contact
with atmospheric pressure and impede the upward and downward movement of water.
As shown in Figure 3.14, the confining layer above the confined aquifer creates a column
of water with very limited exposure to the ground surface. This overburden of water cre-
ates additional pressure within these formations, so any well installed within a confined
aquifer will have a water level higher than the elevation where the water was encountered.
If the water level in the well rises but does not reach the surface, the well is termed an
artesian well . When the pressure in a confined aquifer is sufficient to allow the water level

Flowing
artesian well

Piezometric
surface
(in confined
aquifer)

Artesian well

Confining layer
(impermeable)
Confined aquifer

Unconfined
aquifer

Water table well
(in unconfined aquifer)

Top of the
confined aquifer

FIGURE 3.14
Example of an unconfined and confined aquifer and artesian and flowing artesian wells. (From Environment
Canada, Aquifers and wells, http://www.ec.gc.ca/water/images/nature/gedwtr/a5f3e.htm, 2009.)